Abstract

Given graphs F, H and G, we say that G is (F, H )v‐Ramsey if every red/blue vertex coloring of G contains a red copy of F or a blue copy of H. Results of Łuczak, Ruciński and Voigt, and Kreuter determine the threshold for the property that the random graph G(n, p) is (F, H )v‐Ramsey. In this paper we consider the sister problem in the setting of randomly perturbed graphs. In particular, we determine how many random edges one needs to add to a dense graph to ensure that with high probability the resulting graph is (F, H )v‐Ramsey for all pairs (F, H) that involve at least one clique.

Highlights

  • Given graphs F, H and G, we say that G is (F, H)v-Ramsey if every red/blue vertex coloring of G contains a red copy of F or a blue copy of H

  • For l ∈ N, a sequence of graphs H1, ... , Hl, and a graph G, we say that G is (H1, ... , Hl)v-Ramsey if for every l-coloring of the vertices of G, there is some i ∈ [l] for which G contains a copy of Hi whose vertices are all colored in the ith color

  • Recall that in our algorithmic proof of the 1-statement in Theorem 1.11, we worked in an ε-regular k-tuple in Gn, using the vertex Ramsey properties of the random graph in each part to iteratively grow a red clique or try to build a copy of H

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Summary

INTRODUCTION

For l ∈ N, a sequence of (not necessarily distinct) graphs H1, ... , Hl, and a graph G, we say that G is (H1, ... , Hl)v-Ramsey if for every l-coloring of the vertices of G, there is some i ∈ [l] for which G contains a copy of Hi whose vertices are all colored in the ith color. For l ∈ N, a sequence of (not necessarily distinct) graphs H1, ... Hl, and a graph G, we say that G is Hl)v-Ramsey if for every l-coloring of the vertices of G, there is some i ∈ [l] for which G contains a copy of Hi whose vertices are all colored in the ith color. Hl)-Ramsey if for every l-coloring of the edges of G, there is some i ∈ [l] for which G contains a copy of Hi whose edges are all colored in the ith color. The classical question in Ramsey theory is to establish the smallest n ∈ N such that the complete graph Kn on n vertices is This, the analogous question in the setting of vertex colorings is completely trivial.

Vertex Ramsey properties of random graphs
Randomly perturbed graphs
Some intuition for vertex Ramsey problems in randomly perturbed graphs
Notation
Organization of the paper
The 0-statement
Proof of the 0-statement
The 1-statement
Probabilistic tools
An algorithm for the 1-statement
Proof of correctness
ROBUST RAMSEY PROPERTIES OF RANDOM GRAPHS
Janson’s inequality for a refined random graph
An exponential upper tail bound
CONCLUDING REMARKS
Full Text
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